Monaco retain an option to buy the 23-year-old centre-back, who made 21 appearances for the Blues last term, per report.
“I am very happy to join AS Monaco and to be back in Ligue 1, a league in which I started and which I know well,” Sarr told Monaco’s website.
“I would like to thank the club for its confidence [in me]. I am now looking forward to blending in with my new surroundings and getting to know my new team-mates, with the desire to achieve a lot together.”
Monaco’s sporting director Paul Mitchell added: “It is a great joy for us to welcome Malang to the club.
“His technical profile and versatility will bring new solutions to our defensive unit. At 23 years of age, Malang knows the demands of the top level, as evidenced by his stints with major European clubs such as Chelsea FC and FC Porto.
“In our desire to give to our squad more experience of the biggest games, he has the profile we were looking for and he will be a great help to us to face the demanding schedule that we have. We welcome him!”
